Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA, and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major, low cost, energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually. Additionally or alternatively, CO rich gas streams (syngas) can be produced by gasification of carbonaceous materials, such as coal, petroleum and biomass. Carbonaceous materials can be converted into gas products including CO, CO2, H2 and lesser amounts of CH4 by gasification using a variety of methods, including pyrolysis, tar cracking and char gasification. Syngas can also be produced in a steam reformation process, such as the steam reformation of methane or natural gas.
Catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al., Archives of Microbiology 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is typically associated with co-production of acetate and/or acetic acid. As some of the available carbon is typically converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to GHG emissions.
WO2007/117157, WO2008/115080 and WO2009/022925, the disclosure of which are incorporated herein by reference, describe processes that produce alcohols, particularly ethanol, by anaerobic fermentation of gases containing carbon monoxide. WO2007/117157, describes a process that produces alcohols, particularly ethanol, by anaerobic fermentation of gases containing carbon monoxide. Acetate produced as a by-product of the fermentation process is converted into hydrogen gas and carbon dioxide gas, either or both of which may be used in the anaerobic fermentation process. WO2008/115080, describes a process for the production of alcohol(s) in multiple fermentation stages. By-products produced as a result of anaerobic fermentation of gas(es) in a first bioreactor can be used to produce products in a second bioreactor. Furthermore, by-products of the second fermentation stage can be recycled to the first bioreactor to produce products. WO2009/022925 discloses the effect of pH and ORP in the conversion of substrates comprising CO to products such as acids and alcohols by fermentation.
Fermentation of gaseous substrates can be challenging due to the requirement that at least a portion of the gaseous substrate is dissolved in a typically aqueous fermentation broth before the substrate can be metabolised by a microbial culture. Fermentations involving gaseous substrates, wherein one or more gaseous components are the carbon and optionally the energy source for a microorganism, are particularly challenging due to the large amount of substrate required to be solubilised in a fermentation broth before any metabolism can take place. Examples of gaseous substrates used as a carbon and/or energy source in fermentation include CO, CO2, CH4, H2 and H2S. In particular, sparingly soluble substrates, such as CO and/or H2 require highly efficient mass transfer into an aqueous fermentation broth as CO is both a carbon and energy source for anaerobic fermentation. For example, the theoretical equations for CO and H2 to ethanol are:6CO+12H2→3C2H5OH+3H2O
Thus, six molecules of gas (CO and/or H2) must be dissolved in a fermentation broth to produce one molecule of ethanol.
Mass transfer of a gas into a liquid is a function of three main variables:                1. Concentration Driving Force: The partial pressure of a particular gaseous component is substantially proportional to the rate at which that component can be driven into a solution.        2. Interfacial Surface Area: The larger the interfacial surface area between gas and liquid phases, the higher the opportunity for mass transfer. In particular, the interfacial surface area is typically a function of gas hold-up and bubble size.        3. Transfer Coefficient: The transfer coefficient of a system is influenced by a variety of factors. However, from a practical perspective, typically the largest influence is the relative velocity between the liquid and the gas phases. Relative velocities (and hence mass transfer) are typically increased by increasing turbulence through agitation or other mixing.        
Many devices and equipment for promotion of mass transfer to microorganisms in fermentation of gaseous substrates are known. However, they typically require large amounts of energy in order to attain the necessary mass transfer rates. For example, in order to achieve effective fermentation of CO and optionally H2 into products, such as acid(s) and alcohol(s), the substrate must be made available to the microorganisms in an efficient manner. This is typically achieved by increasing mass transfer rates of CO and optionally H2 molecules into solution by mechanical means, such as vigorous stirring. These methods for increasing mass transfer require a large power input, which becomes inefficient and/or uneconomical as scale increases.
It is an object of the present invention to provide system(s) and/or method(s) that overcomes disadvantages known in the art and provides the public with new methods for the optimal production of a variety of useful products.